This invention relates to improvements in the method of manufacturing photorelief printing plates involving the use of a liquid photopolymer, and to a printing plate manufactured by such improved method. More particularly, this invention relates to a method of manufacturing photorelief printing plates which utilizes registered printing negatives on opposite sides of the photopolymer liquid during the curing of selected portions of the liquid by the exposure of the liquid to an ultraviolet light source which transmits light to the liquid through the non-darkened portions of the negatives, and to a printing plate manufactured by such method. Even more particularly, the invention relates to the manufacture of photorelief printing plates for the flexographic printing process by such a method and to a flexographic printing plate manufactured by such method.
The manufacture of photorelief printing plates by a technique utilizing a liquid photopolymer is known in the prior art, and equipment and supplies for the practice of this technique are commercially available from the Graphic Systems Business Center of Hercules Incorporated in Wilmington, Del. This vendor offers a variety of different Merigraph.RTM. photopolymers for use in this process, with various photopolymers being recommended for use with different types of printing inks. In the system as offered by Hercules, exposure equipment is provided in which a printing negative is placed on a glass screen, a layer of a liquid photopolymer is placed on top of the printing negative, usually with a thin film of a strippable transparent film such as a polypropylene film placed therebetween, a transparent substrate film which will be bonded to the photopolymer by the curing thereof, such as a Mylar polyester film, is placed on the top of the liquid, and the resulting composite structure exposed to an ultraviolet source of light on the other side of the glass screen and to another source of ultraviolet light on the other side of the transparent substrate. This exposure to ultraviolet light will cure the portions of the photopolymer liquid which are not shielded by the darkened portions of the negative on the lightstruck portion of the negative surface and for a finite portion of the depth of the entire substrate surface, and at the same time this will bond the portion of the cured liquid photopolymer which contacts the polyester to such substrate. While this curing is taking place, the portion of the liquid photopolymer which is adjacent to the printing negative and which is non-lightstruck because it is shaded by the darkened portions of the negative will remain in a liquid form for a portion of the depth of the liquid, and upon the removal of the partially manufactured plate from this exposure equipment this uncured liquid is removed to be discarded or recycled, and the removal of this liquid provides the relief in the print surface of the printing plate which is needed for proper printing.